This invention relates to alumina-zirconia ceramic powders having excellent sinterability and a method of making the same. More particularly, this invention provides an excellent starting material for alumina-zirconia ceramic sintered bodies wherein alumina-zirconia ceramic powders having adjusted crystalline phases and a lower content of chlorine ion are obtained by calcining, under specific conditions, composite ceramic powders having dispersed zirconium oxide (zirconia) in aluminum oxide (alumina) based fine particles which are obtained by subjecting aluminum chloride and zirconium chloride as feedstocks to a vapor phase oxidation/pyrolysis process, and wherein when such ceramic powders are used as a starting material to produce molded sintered bodies the sintering initiation temperature can be reduced and the mechanical strength of the sintered bodies can be improved because the transfer and growth of zirconia particles as well as the phase transition of zirconia and alumina can be appropriately adjusted.
In the production of ceramic sintered bodies, the nature of starting powders is important and the control of the crystalline phase of the powders is one of the important factors. In the present invention, good sinterability is afforded by controlling the crystalline phases of the starting powders in the production of sintered bodies comprising alumina and zirconia. We have now found calcination conditions required for affording such crystalline phases. Further improvement in sinterability has been made by preventing the agglomeration during the powder pretreatment, especially by adopting the freeze-drying process. As a result, according to the present invention, the densities of the sintered bodies have become higher and the strengths of the sintered bodies have been greatly improved.
The chemical properties of zirconium are very similar to those of hafnium and therefore it is difficult to separate these two elements. Zirconium usually contains from about 1 to 4 mole % of hafnium. (For example, "7680 Chemical Goods" published by Kagaku Kogyo Nipposha, Japan, pp 155, 1980) Accordingly, when commercially available zirconium chloride is used in producing ceramic powders of the present invention, zirconium chloride used contains from about 1 to 4 mole % of hafnium chloride and zirconium oxide (zirconia) present in ceramic powders also contains from about 1 to 4 mole % of hafnium oxide (hafnia). The behavior of hafnium tetrachloride and that of hafnia obtained by oxidizing hafnium tetrachloride are essentially the same as that of zirconium tetrachloride and of zirconia, respectively. If the content of hafnium is no more than 4 mole %, it may be considered that zirconium containing hafnium exhibits the same behavior as that of pure zirconium in usual use.
When the zirconium component is called merely zirconium oxide or zirconia in the present invention unless otherwise indicated, the zirconium component generally contains no more than 4 mole % of hafnium oxide derived inevitably from the feedstock. Of course, pure zirconium oxide can be used in the present invention.
The toughness of sintered bodies can be significantly improved by finally dispersing zirconia in another ceramic matrix. (For example, "Bulletin of the Ceramic Society of Japan", Vol. 17 (1982), No. 2, pp. 106-111) The reason why toughness is generated is as follows: when sufficiently finely dispersed zirconia paticles are present in the form of the tetragonal phase in a ceramic matrix, these tetragonal zirconia particles transform into the monoclinic phase in the stress field at the tip of propagating cracks and thus absorb energy of crack propagation. In order that zirconia retains the tetragonal phase in the ceramic matrix, its size must be smaller than a critical particle size. For example, when zirconia is present in a dense alumina matrix, it is said that the critical particle diameter of zirconia is approximately 0.5 .mu.m (5000 .ANG.ngstroms). If zirconia particles having a size larger than the critical particle diameter are present in a monoclinic symmetry, the transformation of crystalline phase due to crack propagation does not occur and thus toughness is not enhanced. Accordingly, when the ceramic sintered bodies are toughened by means of such zirconia particles, it is important that zirconia particles are uniformly and finely dispersed.
One of the present inventors has already carried out studies wherein gases of aluminum chloride and zirconium chloride are oxidized in a high temperature flame. Japanese Patent Appln. No. 3336/1983 disclosed that composite ceramic powders having finely dispersed tetragonal zirconia crystallites in alumina particles are produced by simultaneously blowing a mixed gas of aluminum chloride and zirconium chloride into a reaction vessel. The above Patent Application disclosed that the sintered bodies obtained from these powders have high bending strength. This is because the starting powders exhibited the extremely fine dispersion of zirconia in alumina and therefore the resulting sintered bodies also exhibited good dispersion of zirconia.
Studies have been carried out with respect to the densification behavior of these powders and the transitions of crystalline phases of zirconia associated with sintering at atmospheric pressure. Even though the starting powders exhibit uniform dispersion of zirconia, the powder having high content of zirconia cannot afford sintered bodies having desired crystalline phase. When the starting powders having high content of zirconia are sintered at high temperatures, zirconia grains coalesce and grow to a size larger than the critical particle diameter and thus monoclinic zirconia particles are formed. If the sintering temperature can be lowered, the increase of monoclinic zirconia accompanied with the grain growth is suppressed. It has been found effective to ball-mill the composite powders in ethanol with a non-ionic surfactant added, for preventing the agglomeration during the successive drying step and thus for lowering the sintering temperature (S. Hori et al, "Al.sub.2 O.sub.3 --ZrO.sub.2 Ceramics Prepared from CVD Powders", Second International Conference on the Science and Technology of Zirconia, June 21-23, 1983, Stuttgart, West Germany; It will be recorded in "Advances in Ceramics", American Ceramic Society, Vol. 12 (1984)).
In general, if ceramics can be densified at a lower temperature not only the heat energy required for sintering can be saved, but also the mechanical properties of the sintered bodies can be improved. As sintering phenomenon is usually one wherein the disappearance of pores and grain growth occur at the same time, it is desirable if the pores disappear before significant and sometimes harmful grain growth occurs. If the densification is achieved at lower temperatures, therefore, dense sintered bodies having smaller grain sizes are produced. As can be seen from, for example, the data of Passmore et al (E. M. Passmore, R. M. Spriggs and T. Vasilos, "Strength-Grain Size-Porosity Relations in Alumina", J. Am. Ceram. Soc., 48 [1] 1-7 (1965)), the strength of sintered bodies depend largely upon the sintered grain sizes.
In sintered bodies wherein zirconia is dispersed in ceramic matrix, the low temperature sintering can improve the strength of the sintered bodies not only because of the smaller sintered grain sizes, but also because the zirconia particles can remain smaller than the critical particle diameter and retain tetragonal symmetry. Then the toughening effect due to the transformation of zirconia can be effectively utilized. Accordingly, in the case of ceramic powders containing zirconia, it is particularly improtant to improve the sinterability and to achieve the low temperature sintering.
In order to improve the sinterability of the alumina-zirconia composite powders produced by using chlorides as feedstocks, the process as already described was effective wherein the powders were ball-milled in alcoholic solvent with a surfactant. Similar effects were obtained when the alcohol was replaced by water as the solvent.
When such ball-milled powders were dried by means of three processes, i.e., a rotary evaporator drying process, a spray drying process and a freeze drying process, the freeze drying process afforded the best sinterability, the spray drying process afforded the next best sinterability and the rotary evaporation drying process afforded the worst sinterability.
The improvement of sinterability is also achieved by removing residual chlorine ion (Cl.sup.-) from the powders. As can be seen from, for example, C. E. Scott and J. S. Reed, "Effect of Laundering and Milling on the Sintering Behavior of Stabilized ZrO Powders", Am Ceram. Soc. Bull., 58 [6] 587-590 (1979), if Cl.sup.- is present, the initiation temperature of sintering is considerably high and thus a presence of Cl.sup.- is disadvantageous. Following the literature of Scott et al, the removal of Cl.sup.- by means of water washing was carried out by repeating about 6 times the operation wherein the powder was placed in water only by 0.5 wt % to the water, stirred an thereafter centrifuged to separate water and powder. Thus, such a process is extremely inefficient.
Further studies have been carried out with respect to means capable of removing Cl.sup.- other than water washing. As the result, it has been found that not only Cl.sup.- is removed, but also that the transfer and growth of zirconia particles as well as the phase transition of zirconia and alumina occur by means of calcination. It has been also found that good sinterability which could not be achieved by the water washing can be achieved by appropriately controlling the calcination conditions.